Some users may see a device they don't know or recognize labeled as Default in the Authy app. This is not cause for concern, as it is a normal part of our system processes when a phone number is registered for an Authy service before the app is downloaded and configured.
This "default" device is not an actual physical device like a phone or a tablet; It's just a setting we use when creating your account. Creating this "default" device allows us to deliver security codes without you needing to actually configure your physical device(s) to use the Authy application. In fact, if you view the device's details in the Authy app (go to Settings > Devices), you'll see that it's missing all of the information a normal Authy device would have, including latest access, location, device type, and more.